1. Field of the Invention
This invention generally relates to optical communications systems, and more particularly to a system and method for improving the quality of signal transmissions through an optical fiber within a dense wavelength division multiplexing (DWDM) system.
2. Description of the Related Art
Optical communication systems are a substantial and fast-growing constituent of modern communications networks. These systems demonstrate greater bandwidth, lower power requirements, and lower-bit error rates than conventional, non-optical transmission systems. Because of their superior performance, optical systems are highly desirable have shown that they are better suited to high-speed data, video, voice, and other integrated transmissions.
One goal shared by optical engineers in designing optical communication systems is to increase the data transmission capacity of the fiber-optic transmission lines. Wavelength division multiplexing (WDM) has emerged as a particularly favored technology for this purpose. In a WDM system, a plurality of optical channels are carried over a single optical fiber. This is made possible by assigning each channel a different optical wavelength; each of the several wavelengths being transmitted within the optical fiber simultaneously. When different transmitter-receiver pairs are connected to the optical fiber, each pair being tuned to a different wavelength, the overall optical system can support multiple optical channels on the optical fiber. Such an increase in data throughput makes WDM suitable for use in a variety of high-speed telecommunications applications, including cable television, and world-wide-web access.
The performance of a digital receiver in a fiber-optic communications system can be measured in terms of its bit-error rate (BER). This rate expresses the percentage of bits received in error, for example by an optical receiver, compared to the total number of bits received. As an example, a bit-error rate of 10−6 means that, on average, one bit error occurs per million bits received. In typical optical communications systems, BERs of 10−13 are achievable as an operating requirement, although much lower BERs are possible. Since any improvement in the BER of an optical system directly affects the performance efficiency of the overall system, it is a common design objective of optical communications systems to achieve the lowest possible BER.
Numerous approaches not directly related to the measurement of BERs have been taken to improve signal transmission quality in optical communications systems. Traditionally, the optical signal to noise ratio (OSNR) for an optical channel has been used to approximate or quantify the BER for that channel as a result of the known relationships between OSNR and BER. See P. C. Becker, N. A. Olsson, J. R. Simpson, “Erbuim-Doped Fiber Amplifiers,” Academic Press, 1997, p.216. The OSNR value represents a measure of the relative strength of an optical transmission signal as compared to the background noise present in the signal. The OSNR can be typically measured by expensive hardware, such as an optical spectrum analyzer which adds additional, expensive equipment to the system, in combination with specially designed software. This hardware and software is often used within different repeater stages of the optical transmission system and is used to evaluate the overall optical signal spectrum and quantify the optical signal through a comparative analysis of the signal's spectral position and power (strength) vis-a-vis the signal's background noise. As an example, if the OSNR for a particular optical channel is measured to be below a certain, acceptable threshold value, one of the optical parameters of that optical channel, such as the optical channel power, may be adjusted until the OSNR (and thus BER) is increased.
However, the relationship between the OSNR and the BER is imperfect. In particular, the OSNR is unaffected by certain optical properties of the transmission channel that can result in bit errors that directly affect the BER. For example, the chromatic dispersion and polarization mode dispersion of an optical signal do not affect the signal amplitude and hence do not affect the OSNR. These dispersive characteristics do, however, affect the bit errors experienced within an optical channel in a DWDM optical system since they lead to pulse spreading.
Thus the need exists for an inexpensive and easily implemented measure of the BER of an optical signal channel within an optical communications system. Such a measure should avoid the deficiencies of approximating the BER with a measure of the OSNR so that a direct and measurable improvement in the transmission quality of optical channel signals is achieved.